Since the advent of high temperature water tube boilers which burn fuels having substantial slag content, and also with the adoption of certain high temperature processing-type heat exchangers, the removal of adherent deposits from the fire side surfaces has been an increasingly severe problem. Sootblowers employing jets of steam and/or air cannot remove some such deposits. It has long been known that jets of water can be used to assist in slag removal, and it was also understood for many years that the thermal shock and resultant embrittlement of the slag caused by a water jet, combined with the energy of the jet itself, could often dislodge slag not removable from a steaming boiler by other means. However, until the advent of the so-called constant jet progression system disclosed in U.S. Pat. No. 3,782,336 granted Jan. 1, 1974 to J. E. Nelson, it was frequently impractical to use water jets for this purpose, because it was not possible to control and limit the thermal shock to a value which would avoid premature failure of the tubes.
Although the aforementioned constant jet progression system employs a high velocity concentrated jet, and a considerable proportion of the water actually bounces off the tubes or slagged surfaces during the operation of that system, some chilling and embrittlement of the slag inherently result from the impingement of the jet, and some cracking may occur which assists the kinetic effect of the jet in dislodgment of the slag. The Nelson constant jet progression system, however, permitted such a reduction in the amount of water employed and of the amount of water which remained in contact with the tubes (or slag) while vaporizing, that the total amount of rapid heat extraction resulting from the chilling and latent heat of vaporization was, for the first time, reliably reduced to safe levels. In fact in normal use the Nelson system reduced the thermal shock effect to levels very much below the maximum which could be tolerated without danger of premature tube failure, and frequently no substantial or observable cracking of the slag occurred prior to its actual dislodgment.
The present invention aims to provide an improved method and apparatus whereby highly heated slag can be dislodged even more rapidly, by means involving accurate application of a controlled amount of thermal shock in conjunction with the kinetic energy derived from a separately applied pulsed high velocity jet.
A related object of the invention is to provide an improved system wherein a pulsed high velocity jet of water impacts the slagged surface while being moved thereover at a controlled rate of progression and wherein prior to being impacted by each pulse of such jet the increment of the surface which is to be impacted is first chilled to a limitd degree by a quenching stream which tends to cause fissures into which water is forced by the pulsed high velocity jet to promote dislodgment of the slag by expansion of water in the fissures.
Other objects and advantages of the invention will become apparent to persons skilled in the art upon consideration of the present disclosure in its entirety.